Process for the preparation of an acetonitrile derivative

ABSTRACT

Process for the preparation of di-n-propyl acetonitrile of the formula: ##STR1## whereby, in a single step, sodium n-propylate in n-propanol medium is added to a reaction medium which is formed of a cyanacetate of general formula: ##STR2## in which R represents an alkyl radical having from 1 to 4 carbon atoms, and n-propyl bromide or iodide, the alkylation reaction taking place under reflux, the crude ester obtained is saponified with a 10 to 20% solution of sodium hydroxide or potassium hydroxide, the resulting salt is acidified with a strong acid, to give crude di-n-propyl cyanacetic acid, which is decarboxylated by heating at a temperature between 140° C. and 190° C., so as to obtain the di-n-propyl acetonitrile.

This application is a continuation of Ser. No. 800,346 filed May 25,1977, now abandoned.

The present invention relates generally to a novel process for thepreparation of an acetonitrile derivative and also to the derivativeobtained by this process.

The invention is particularly concerned with a novel process for thepreparation of di-n-propyl acetonitrile of formula: ##STR3##

Di-n-propyl acetonitrile is a known product which is of particularinterest for the preparation of compounds having pharmacologicalproperties. For example, di-n-propyl acetonitrile can be used for thepreparation of di-n-propyl acetamide, which has extremely valuableneuropsychotropic properties, as described in B.S.M. (French SpecialMedicament Patent) No. 2442 M.

Di-n-propyl acetamide can be easily prepared with excellent yields, ofthe order of 83%, when starting from the di-n-propyl acetonitrile, byhydrolysing this latter compound, for example, by means of an aqueoussolution of 75to 80% sulphuric acid and at a temperature between 80° and130° C.

The conventional processes for the preparation of di-n-propylacetonitrile are generally complicated and necessitate the use ofreactants which are dangerous for the manufacturing personnel. Forexample, the preparation of di-n-propyl acetonitrile, when starting fromdi-n-propyl ketone, requires the use of sodium cyanide, which is anextremely toxic product.

Moreover, certain phases in the preparation consist in a hydrogenation,which is always difficult to carry out on the industrial plane.

The need for finding an industrial process for obtaining di-n-propylacetonitrile is thus of paramount importance.

Hitherto, the synthesis of acetonitrile substituted in the α-position bytwo propyl groups, starting from an ester of cyanacetic acid, has onlybeen subject to experimentation in the case where each of the two propylgroups is an isopropyl group.

In this connection, mention may be made of the processes described byMARSHALL [J. Chem. Soc., 2754-2761 (1930)], by BROWN and collaborators[J. Am. Chem. Soc., 77, 1083-1089 (1955)] and by NEWMAN andcollaborators [J. Am. Chem. Soc., 82, 873-875 (1960)].

These processes are characterised by a succession of three or four quitedistinct stages or steps, starting from an ester of cyanacetic acid,namely:

an alkylation phase, which is common to all three processes, for thepurpose of obtaining a diisopropyl cyanacetic ester,

a phase for elimination of the monoalkylated ester,

a phase for saponification of the diisopropyl cyanacetic ester in thecase of the processes proposed by MARSHALL and NEWMAN and collaborators,

and a decarboxylation phase, either of the diisopropyl cyanacetic esterin the case of the process proposed by BROWN and collaborators, or ofthe diisopropyl cyanacetic acid in the case of the processes proposed byMARSHALL and by NEWMAN and collaborators.

Thus, MARSHALL prepares diisopropyl acetonitrile from a cyanaceticester, by treating with sodium an alcoholic solution of this ester andby causing this mixture to react for several hours with an excess ofisopropyl iodide. The monoalkylated product is eliminated by means of a10% sodium hydroxide solution and the crude dialkyl ester obtained bythis procedure is then treated with a 35% potassium hydroxide solutionfor 16 hours. After acidification, the diisopropyl cyanacetic acidobtained is decarboxylated by distillation in the presence of twice itsweight of molten potassium hydroxide.

BROWN and collaborators, for their part, obtain diisopropyl acetonitrilefirst of all by treating, with isopropyl iodide, a solution ofcyanacetic ester in n-propanol containing sodium n-propylate, this beingeffected by refluxing for 2 hours, and then by again adding sodiumn-propylate in n-propanol and isopropyl iodide. The reaction medium isonce again heated under reflux for 3 hours, the monoalkylated product iseliminated by a 10% sodium hydroxide solution and the diisopropylcyanacetic ester is then distilled several times in the presence oftwice its weight of potassium hydroxide.

Finally, NEWMAN and collaborators prepare diisopropyl acetonitrile byfirst of all carrying out a reaction, under reflux for 3 hours, of ethylcyanacetate with isopropyl iodide in the presence of sodium ethylate inethanolic medium, further adding sodium ethylate and then isopropyliodide and once again heating the reaction medium under reflux for 3hours. After again adding sodium ethylate and then isopropyl iodide andheating for 2 hours under reflux, the disopropylated derivative obtainedis washed with a 15% potassium hydroxide solution and then hydrolysed bymeans of an alcoholic solution of 35% potassium hydroxide under refluxfor 26 hours and the diisopropyl cyanacetic acid is heated to 180°-200°C. in the presence of copper powder.

In view of the great similarity as regards chemical structure betweendiisopropyl acetonitrile and di-n-propyl acetonitrile, attempts havebeen made to prepare this latter compound by applying the aforementionedprocesses used for the preparation of the diisopropyl acetonitrile.

Tests carried out with the technique proposed by MARSHALL only producedinsignificant yields of pure di-n-propyl acetonitrile, of the order of20%, if each synthesis intermediary is purified, or 35%, if eachintermediary is used in the crude state, these yields being calculatedon the basis of the initial cyanacetic ester. Furthermore, theintermediate products prepared in this process are contaminated withimpurities, which prevent their use in the crude state. Thus, the crudedi-n-propyl cyanacetic acid obtained according to MARSHALL, or accordingto NEWMAN and collaborators, is found to be contaminated by 18 to 25%,and 32 to 34%, respectively, of a product which seems to be adi-n-propyl formamidoacetic ester.

Furthermore, the procedure proposed by BROWN and collaborators, as itnecessitates a double alkylation phase, has proved to be inadequate forthe preparation of di-n-propyl acetonitrile. In effect, this product hasbeen obtained in pure form with yields which vary from 28 to 44%,calculated from the initial methyl cyanacetate.

Finally, the process proposed by NEWMAN and collaborators, whichnecessitates a treble alkylation phase and is particularlytime-consuming, only provided yields in the region of 40% of puredi-n-propyl acetonitrile, calculated on the basis of the initialcyanacetic ester. It has also been observed that the saponification ofthe di-n-propyl cyanacetic ester leads to a mixture of 10% ofdi-n-propyl acetic acid and 5% of di-n-propyl acetic amide.

In conclusion, all of the aforesaid methods, applied to the preparationof di-n-propyl acetonitrile, are essentially distinguished by theircomplexity and their considerable duration, by the impurities obtainedat the different stages, necessitating the elimination of suchimpurities for the subsequent stages, and by the poor yields of thefinal di-n-propyl acetonitrile.

Consequently, it was essential to find a process for the preparation ofdi-n-propyl acetonitrile which has the following qualities:

simplicity as regards procedure,

shorter overall duration,

higher yields,

a production cost which is as low as possible, so that it can be validlyused on the industrial scale.

In accordance with the present invention, it has now been discoveredthat di-n-propyl acetonitrile can be obtained in accordance with such aprocess which can be used industrially, starting from a cyanaceticester.

Thus in accordance with the process of the invention, di-n-propylacetonitrile is prepared by reacting, in one single stage and in an-propanol medium, an ester of cyanacetic acid of the general formula:##STR4## in which R represents an alkyl radical having from 1 to 4carbon atoms, preferably a methyl or ethyl radical, with n-propylbromide or n-propyl iodide in the presence of sodium n-propylate, thenby saponifying the crude ester thus obtained with a 10 to 20% by weightsolution of potassium hydroxide or sodium hydroxide and by acidifyingthe salt thus formed with a strong acid, such as for examplehydrochloric acid, to obtain the crude di-n-propyl cyanacetic acid,which is decarboxylated by heating to a temperature between 140° C. and190° C., this yielding the di-n-propyl acetonitrile.

The starting-products of formula II are either known products which havebeen mentioned in the foregoing publications, or products which can beobtained by known methods.

As regards the alkylation phase, the reactants are utilised by adding,at a temperature between 45° C. and 55° C., the sodium n-propylate inn-propanol medium to a reaction medium which comprises the cyanaceticester and the n-propyl halide. The alkylation reaction is then carriedout under reflux for about 3 hours.

Saponification of the crude di-n-propyl cyanacetic ester is preferablycarried out at a temperature between 60° and 70° C. over a period of 3hours in the proportion of 1.25 to 2 mols of hydroxide/mol of ester, andthe subsequent acidification is effected, for example, with a 36%hydrochloric acid solution, at a temperature slightly lower than 40° C.

In accordance with an alternative procedure, the saponification phasecan be carried out in presence of a quaternary ammonium salt such as,for example, trimethyl cetylammonium bromide, benzyl trimethyl ammoniumchloride or lauryl trimethyl ammonium bromide. The concentration ofquaternary ammonium salt may vary from 0.005 mol to 0.1 mol/mol ofdi-n-propyl cyanacetic ester. Temperature as regards saponification andthe time necessary for this operation will vary as a function of thequantity of quaternary ammonium salt used.

For a concentration of quaternary ammonium salt of 0.1 mol/mol of ester,saponification will take place for 3 hours at 30° C., and for aconcentration of 0.005 mol/mol of ester, the operation will be completedin 1 hour at 60° to 65° C.

As regards the decarboxylation phase, this latter will be carried out onthe crude di-n-propyl cyanacetic acid at a temperature between 140° and190° C. and preferably between 175° and 190° C.

In accordance with a modification of this last operation, thedecarboxylation of the di-n-propyl cyanacetic acid can be carried out inone continuous phase. After the acid concerned is brought to atemperature of 185°-190° C. and the decarboxylation reaction initiated,di-n-propyl cyanacetic acid is continuously introduced, withsimultaneous elimination of the liberated carbon dioxide gas and of thedi-n-propyl acetonitrile which forms.

The process of the invention provides indisputable advantages ascompared with the processes disclosed in the previously mentioned priorart.

In the first place, the process of the invention offers the possibilityof obtaining considerable yields of pure di-n-propyl acetonitrile, theyields being at least 80% as compared with the initial cyanacetic ester,whereas with the processes suggested by the prior art, it has not beenpossible to obtain yields higher than 50% with respect to the samestarting ester.

In addition, the process of the invention is definitely more simple thanthose of MARSHALL, BROWN and collaborators, or NEWMAN and collaborators,referred to above. For example, the process of the invention permits thealkylation phase to be carried out in one single operation, comprising asingle use of the n-propyl halide and alkali metal n-propylate.

By contrast, the process proposed by BROWN and collaboratorsnecessitates two successive additions of alcoholate and of halide, whilein accordance with the process proposed by NEWMAN and collaborators theaddition of alcoholate and of halide is carried out in three successiveoperations for each product.

The times necessary for the alkylation and saponification phases arealso considerable in the case of the known processes: at least 8 hoursfor the alkylation phase according to the process proposed by NEWMAN andcollaborators, and 26 hours for the saponification phase, according tothese same authors.

The process of the invention, on the contrary, enables the correspondingalkylation and saponification phases to be effected much more quicklythan by means of the known processes.

As regards the saponification phase, the time which is necessary forthis operation will be advantageously reduced in the presence of aquaternary ammonium salt, for example, the trimethyl cetylammoniumbromide. This quaternary ammonium salt offers in addition the advantageof reducing the danger of hydrolysis of the nitrile function of thedi-n-propyl cyanacetic ester.

Furthermore, the decarboxylation phase of the known processes involvesthe necessity, apart from a raising of the temperature, of adding asupplementary product, either potassium hydroxide or copper powder.

According to the invention, the decarboxylation phase occurs simply byheating the di-n-propyl cyanacetic acid.

An additional disadvantage presented by the processes suggested by theprior art, and more especially by the alkylation phases envisaged inthese processes, is concerned with the recovery of the solvent, of thereactants which have not reacted and of the by-products formed duringthe reaction.

This recovery, which is fairly difficult when using sodiumethylate/ethanol or sodium methylate/methanol is facilitated by the useof the sodium n-propylate/n-propanol pair, which provides greaterpossibility of separation by distillation of the unreacted n-propylhalide, of the ether formed during the reaction and of the alcohol whichmay be liberated by transesterification of the cyanacetic ester by then-propanol.

All these disadvantages, presented by the processes suggested by theprior art, increase the quantity of material to be used, the labourforce and the energy consumption, causing a concurrent increase in thecost of production.

Among the disadvantages presented by the known processes, the presenceof harmful impurities at the different stages is certainly not the leastnegligible.

These impurities, which are present at each phase of the process,singularly complicate the successful performance of the said process.Consequently, it is necessary for them to be eliminated at each stage,thus considerably increasing the intermediate handling operations, whichare always costly at the industrial level.

For example, the processes suggested by the prior art envisage theelimination of the monoalkylated product after the alkylation phase,this being effected by means of 10% potassium hydroxide.

The alkylation phase as envisaged within the scope of the processaccording to the invention renders unnecessary the intermediatepurification of the di-n-propyl cyanacetic ester, which may be used inits crude form.

It has, in fact, been observed that the use of the alkylation reactantsaccording to the invention, depending essentially on the introduction ofsodium n-propylate/n-propanol into a medium formed by the ester offormula II and the n-propyl halide, provides the particular advantage ofavoiding to a maximum extent the formation of monopropyl cyanaceticester, which is much greater when the n-propyl halide is added to thecyanacetic ester/sodium-n-propylate mixture. This monopropyl cyanaceticester, does, in fact, eventually lead to the formation of valeronitrile,which is a particular nuisance and must be eliminated.

The use of the alkylation reactants in accordance with the inventionpermits the content of valeronitrile in the final di-n-propylacetonitrile to be very substantially reduced, this content passing fromapproximately 3.6% to only 0.3% according to the invention.

Furthermore, the use of sodium n-propylate/n-propanol in accordance withthe invention has been found to be much more advantageous than the useof sodium ethylate/ethanol or the use of sodium methylate/methanol, asproposed in the processes according to the prior art.

It has, in fact, been established that the content of monopropylcyanacetic ester in the crude di-n-propyl cyanacetic ester, whichsubsequently leads to valeronitrile, is increased, and can even varyfrom 2 to 5% if the reflux temperature of the reaction medium is too lowat the time of the alkylation phase, which is the case with methanol orethanol.

It has also been found that the use of the sodium ethylate/ethanol paircan give rise to the formation of a not inconsiderable quantity, in theregion of 1%, of n-propyl cyanacetic ethylate at the time of thealkylation phase.

Moreover, as previously mentioned, the saponification of the crudedi-n-propyl cyanacetate in accordance with the conditions proposed byNEWMAN and collaborators, or by MARSHALL, that is to say, by means of35% potassium hydroxide for 16 to 26 hours, leads to the formation of acrude di-n-propyl cyanacetic acid containing from 18 to 34% of animpurity, which seems to be a di-n-propyl formamidoacetate and has to beeliminated. This last product does not, in fact, give di-n-propylacetonitrile by decarboxylation, but di-n-propyl acetamide.

Yet again, the process according to the invention avoids thisdisadvantage and, at the same time, an intermediate purification of thecrude di-n-propyl cyanacetic acid.

During tests carried out within the scope of the present invention,attempts have been made to combine certain phases characteristic of theprocess of the invention with phases which are used by the previouslymentioned prior processes.

For example, the dialkylation phase of the process according to theinvention, combined with the decarboxylation stage of the di-n-propylcyanacetic acid by being melted with twice its weight of 85% potassiumhydroxide, at a temperature between 190° and 360° C., in accordance withthe procedure proposed by MARSHALL, only supplied 11% of di-n-propylacetonitrile with respect to the cyanacetic ester used. In this methodof procedure, most of the di-n-propyl cyanacetic acid was transformedinto di-n-propyl acetamide and di-n-propyl acetic acid.

A variation of the decarboxylation process proposed by MARSHALL has alsobeen carried out with di-n-propyl cyanacetic acid, obtained according tothe process of the invention, and twice its weight of 98% sodiumhydroxide. This mixture, distilled for 21/4 hours at 370° C., onlysupplied 38.3% of di-n-propyl acetonitrile with respect to thedi-n-propyl cyanacetic acid used.

Furthermore, the methyl di-n-propyl cyanacetate obtained in accordancewith the process of the invention, was distilled in the presence ofpotassium hydroxide, following the procedure of BROWN and collaborators.

By using twice as much by weight of 97.7% potassium hydroxide as ofester and by heating to 380° C. for at least 21/4 hours, only 28.4% ofpure di-n-propyl acetonitrile, relatively to the initial cyanacetate,were obtained.

A similar test, carried out with the same quantity of 98% sodiumhydroxide, under the same conditions as regards temperature andduration, provided a yield of 44.4% of di-n-propyl acetonitrilerelatively to the initial cyanacetate.

From all the results set out above, it is obvious that the processaccording to the invention constitutes an undoubted advantage over theprocesses suggested by the prior art.

Furthermore, the process of the invention has proved to be superior tothe known process as used for preparing di-n-propyl acetonitrile, whichprocess has been previously referred to.

The invention is illustrated by the following non-limiting Examples:

EXAMPLE 1 Preparation of di-n-propyl acetonitrile (a) Di-n-propylcyanacetic acid

First of all, a sodium n-propylate solution was prepared from 7.42 g(0.322 mol) of sodium and 180 ml of anhydrous n-propanol, by heatingwith gentle reflux until complete dissolution of the sodium.

Into a 500 ml spherical flask, equipped with a dropping funnel, amechanical stirrer, a thermometer and a condenser, above which wasdisposed a calcium chloride trap, were introduced 16.95 L g (0.141 mol)of ethyl cyanacetate and 40.69 g (0.33 mol) of n-propyl bromide. Thismixture was heated to 45° C. and then there was added thereto, slowlyand while stirring, the previously prepared solution of sodiumn-propylate, keeping the temperature of the reaction medium at 50°-55° lC. by gentle external cooling.

With the completion of the operation of introduction, the mixture wasbrought to reflux temperature in 30 minutes and kept at this temperaturefor 3 hours. The n-propanol was then distilled and the distillationstopped when the temperature of the residual mass had reached 115° C.

The crude ester obtained in this way was then treated with a solution of7.5 g of flaked sodium hyroxide in 67.5 ml of water. The mixture wasintroduced into a 250 ml spherical flask, equipped with a condenser, andthen the reaction medium was slowly brought to 60°-70° C. Thistemperature was maintained for 3 hours, whereafter the mixture wascooled to about 50° C. and the ethanol which had formed and the residueof n-propanol were eliminated under a pressure of 70 mm.Hg. The solutionthus obtained was cooled to 20° C. and acidified, while stirring, byaddition of 26.25 g of 36% hydrochloric acid. During this operation, thetemperature of the reaction medium was kept below 40° C. by cooling.Stirring was continued for 30 minutes, whereafter the mixture was leftstanding for 30 minutes. The oily layer of di-n-propyl cyanacetic acidwas decanted and the aqueous phase extracted with 35 ml of toluene. Theextract in toluene was then added to the decanted di-n-propyl cyanaceticacid, whereafter the solution in toluene was washed, in a separationfunnel, with a solution of 1.5 g of sodium chloride in 14 ml of water.The toluenic phase was decanted and the toluene distilled underatmospheric pressure.

Using this procedure, 25 g of crude di-n-propyl cyanacetic acid wereobtained.

(b) Di-n-propyl acetonitrile

Into a 100 ml spherical flask fitted with a thermometer and a condenserwere introduced 25 g of crude di-n-propyl cyanacetic acid obtained bythe method previously described, and the mixture was heated on an oilbath.

Decarboxylation commenced at a temperature in the region of 140° C. Themixture was then brought to reflux temperature, that is to say, to about160° C. and then to 190° C. in 2 hours. This temperature was maintaineduntil the release of gas was completed, this taking 2 hours. Thedi-n-propyl acetonitrile thus formed was then slowly distilled and thefraction passing over between 165° C. and 175° C. was collected. Asecond distillation was then carried out.

Using this procedure, 14.7 g of di-n-propyl acetonitrile were collected.B.P.:170° C.

Yield: 83%, relatively to the ethyl cyanacetate used.

EXAMPLE 2 Preparation of the di-n-propyl acetonitrile (a) Di-n-propylcyanacetic acid

Initially, a solution of sodium n-propylate was prepared from 50 g (2at.g +10%) of sodium and 804 g (1000 ml) of anhydrous n-propanol, byheating to 50°-55° C. for 60 to 90 minutes.

99.1 g (1 mol) of methyl cyanacetate and 270.6 g (2.2 mols) of n-propylbromide were introduced into a 2-liter spherical flask. While stirring,the mass was brought to 45°-50° C. and, at this temperature, thesolution of sodium n-propylate in propanol was regularly introduced.This operation lasted from 60 to 75 minutes.

When the operation of introduction was completed, the mixture wasrefluxed for 3 hours. The n-propanol was then distilled until atemperature of 120°-125° C. was reached in the residual mass. The crudeester obtained was then treated with 500 g of a 10% aqueous solution ofsodium hydroxide and with 0.36 g of cetyl trimethyl ammonium bromide.

The mixture was brought to reflux for 1 hour, was then cooled to about50° C., and thereafter the residual alcohols were eliminated underreduced pressure (50 to 100 mm.Hg).

The solution obtained was cooled and then acidified, without exceeding40° C., by means of 175 g of 36% hydrochloric acid. The mixture wasmaintained in this state for 30 minutes and then the di-n-propylcyanacetic acid was decanted. The lower aqueous layer was extracted with250 g of toluene. The two organic phases were combined, washed once with100 g of purified water and the solvent eliminated by distillation underreduced pressure, to obtain 154.5 g of crude di-n-propyl cyanaceticacid.

(b) Di-n-propyl acetonitrile

The previously obtained crude di-n-propyl cyanacetic acid wastransferred into a 250 ml spherical flask and progressively brought toreflux, while eliminating the last traces of toluene by means of aDean-Stark system, until a temperature of the mass in the region of 175°to 180° C. was obtained. Decarboxylation started in the region of 140°C. and the reaction was practically complete after 1 hour of reflux. Themixture was kept for a total of 2 hours under reflux. The masstemperature reached 205°-210° C. in the first few minutes of therefluxing operation, and dropped down again and became stable in theregion of 185° C. The mixture was then distilled at atmosphericpressure.

In this manner, 102.5 g of di-n-propyl acetonitrile were recovered.Yield of crude product: 82%, relatively to the methyl cyanacetate.

Yield of pure product: 80%.

EXAMPLE 3 Preparation of di-n-propyl acetonitrile

Into a 50-liter enamelled container were introduced 30 kg of di-n-propylcyanacetic acid. While stirring, heating under reflux to 185°-190° C.was carried out and the temperature was maintained as such for 15minutes. The di-n-propyl acetonitrile thus formed was distilled, while69.4 kg of di-n-propyl cyanacetic acid were continuously introduced.

The speed of introduction was regulated as a function of the speed ofdistillation of the nitrile, while the temperature of the mass wasmaintained at 185°-190° C. The operation of introduction lasted forabout 41/2 hours, during which 40.9 kg of crude di-n-propyl acetonitrilewere recovered. Distillation was continued by gradually raising thetemperature of the mass to 206° C. and until the operation wascompleted. This operation lasted 6 hours, during which there wererecovered 16.350 kg and then a further 8.980 kg of crude di-n-propylacetonitrile.

The apparatus was brought under reduced pressure (about 100 mm.Hg) and anew fraction of 1.640 kg of di-n-propyl acetonitrile was collected.

Using this procedure, 67.87 kg of crude di-n-propyl acetonitrile wereobtained.

We claim:
 1. Process for the preparation of di-n-propylacetonitrile ofthe formula: ##STR5## whereby, (a) sodium n-propylate in n-propanolmedium is added to a reaction medium which is formed of a cyanacetate ofgeneral formula: ##STR6## in which R represents an alkyl radical havingfrom 1 to 4 carbon atoms, and n-propyl bromide or iodide, the alkylationreaction taking place under reflux at atmospheric pressure, (b) thecrude ester obtained is saponified at a temperature between 30° and 70°C. with a 10 to 20% by weight solution of sodium hydroxide or potassiumhydroxide in the proportion of 1.25 to 2 mols of sodium or potassiumhydroxide per mol of crude ester, the resulting salt is acidified with astrong acid at a temperature not exceeding 40° C., to give crudedi-n-propyl cyanacetic acid, (c) which is decarboxylated solely byheating, in the absence of any supplementary agent, at a temperaturebetween 140° C. and 190° C., so as to obtain thedi-n-propylacetonitrile.
 2. Process according to claim 1, wherein thecyanacetate is methyl cyanacetate or ethyl cyanacetate.
 3. Processaccording to claim 1, whereby the addition of sodium n-propylate iscarried out when the temperature of the reaction medium is at 45° C. to55° C.
 4. Process according to claim 1, whereby the saponification iseffected in the presence of a quaternary ammonium salt.
 5. Processaccording to claim 4, wherein the quaternary ammonium salt is trimethylcetylammonium bromide.
 6. Process according to claim 4, whereby thesaponification is carried out in the presence of 0.005 to 0.1 mol ofquaternary ammonium salt mol of crude ester.
 7. Process according toclaim 1, whereby the acidification takes place by means of 36%hydrochloric acid at a temperature which does not exceed 40° C. 8.Process according to claim 1, whereby the decarboxylation takes place ata temperature which is between 175° and 190° C.
 9. Process according toclaim 1, whereby the decarboxylation operation is carried out bycontinuously transferring the di-n-propyl cyanacetic acid into thedecarboxylation medium and by simultaneously eliminating the formeddi-n-propyl acetonitrile.